1. Field of the Invention
The present invention relates to an oxadiazole derivative, and a light-emitting element, a light-emitting device, an electronic device and a lighting device each using the oxadiazole derivative.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence have been extensively conducted. In the basic structure of such a light-emitting element, a layer including a light-emitting substance is interposed between a pair of electrodes. By applying a voltage to this element, light emission can be obtained from the light-emitting substance.
Since this type of light-emitting element is a self-luminous type, it has advantages over a liquid crystal display in that visibility of a pixel is high and that no backlight is needed. Therefore, light-emitting elements are thought to be suitable as flat panel display elements. Further, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight and that response speed is very high.
Further, since this type of a light-emitting element can be formed to have a film shape, surface light emission can be easily obtained. Therefore, a large-area element using the surface light emission can be formed. This feature is difficult to realize with point light sources typified by a filament lamp and an LED or with linear light sources typified by a fluorescent light. Therefore, such light-emitting elements also have a high utility value as surface light source that can be applied to lighting apparatuses or the like.
Light-emitting elements using electroluminescence are broadly classified according to whether their light-emitting substance is an organic compound or an inorganic compound. When an organic compound is used as a light-emitting substance, by application of a voltage to a light-emitting element, carriers (i.e., electrons and holes) are injected into a layer including the light-emitting organic compound from a pair of electrodes, whereby a current flows. Then, the carriers recombine to place the light-emitting organic compound into an excited state. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light.
An improvement in characteristics of such light-emitting elements involves a large number of material-dependent problems. To solve them, developments in element structures and materials and the like have been made.
For instance, as an electron-transport material for light-emitting elements, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) has been widely used (see Non-Patent Document 1). Yet, commercialization of light-emitting elements requires a further reduction in drive voltage. Thus, new materials have been extensively studied to improve element characteristics.